[PATCH v3 04/35] x86/bugs: Restructure mds mitigation

David Kaplan
2025-01-08T20:25:46Z

Restructure mds mitigation selection to use select/update/apply
functions to create consistent vulnerability handling.

Signed-off-by: David Kaplan <david.kaplan@amd.com>
---
arch/x86/kernel/cpu/bugs.c | 70 +++++++++++++++++++++++++++++++++-----
1 file changed, 62 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 592d40551432..ff2d6f2e01f4 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -34,6 +34,25 @@

#include "cpu.h"

+/*
+ * Speculation Vulnerability Handling
+ *
+ * Each vulnerability is handled with the following functions:
+ * <vuln>_select_mitigation() -- Selects a mitigation to use. This should
+ * take into account all relevant command line
+ * options.
+ * <vuln>_update_mitigation() -- This is called after all vulnerabilities have
+ * selected a mitigation, in case the selection
+ * may want to change based on other choices
+ * made. This function is optional.
+ * <vuln>_apply_mitigation() -- Enable the selected mitigation.
+ *
+ * The compile-time mitigation in all cases should be AUTO. An explicit
+ * command-line option can override AUTO. If no such option is
+ * provided, <vuln>_select_mitigation() will override AUTO to the best
+ * mitigation option.
+ */
+
static void __init spectre_v1_select_mitigation(void);
static void __init spectre_v2_select_mitigation(void);
static void __init retbleed_select_mitigation(void);
@@ -41,6 +60,8 @@ static void __init spectre_v2_user_select_mitigation(void);
static void __init ssb_select_mitigation(void);
static void __init l1tf_select_mitigation(void);
static void __init mds_select_mitigation(void);
+static void __init mds_update_mitigation(void);
+static void __init mds_apply_mitigation(void);
static void __init md_clear_update_mitigation(void);
static void __init md_clear_select_mitigation(void);
static void __init taa_select_mitigation(void);
@@ -165,6 +186,7 @@ void __init cpu_select_mitigations(void)
spectre_v2_user_select_mitigation();
ssb_select_mitigation();
l1tf_select_mitigation();
+ mds_select_mitigation();
md_clear_select_mitigation();
srbds_select_mitigation();
l1d_flush_select_mitigation();
@@ -175,6 +197,14 @@ void __init cpu_select_mitigations(void)
*/
srso_select_mitigation();
gds_select_mitigation();
+
+ /*
+ * After mitigations are selected, some may need to update their
+ * choices.
+ */
+ mds_update_mitigation();
+
+ mds_apply_mitigation();
}

/*
@@ -229,9 +259,6 @@ static void x86_amd_ssb_disable(void)
wrmsrl(MSR_AMD64_LS_CFG, msrval);
}

-#undef pr_fmt
-#define pr_fmt(fmt) "MDS: " fmt
-
/* Default mitigation for MDS-affected CPUs */
static enum mds_mitigations mds_mitigation __ro_after_init =
IS_ENABLED(CONFIG_MITIGATION_MDS) ? MDS_MITIGATION_AUTO : MDS_MITIGATION_OFF;
@@ -277,12 +304,20 @@ enum rfds_mitigations {
static enum rfds_mitigations rfds_mitigation __ro_after_init =
IS_ENABLED(CONFIG_MITIGATION_RFDS) ? RFDS_MITIGATION_AUTO : RFDS_MITIGATION_OFF;

+/* Return TRUE if any VERW-based mitigation is enabled. */
+static bool __init verw_mitigation_enabled(void)
+{
+ return (mds_mitigation != MDS_MITIGATION_OFF ||
+ (taa_mitigation != TAA_MITIGATION_OFF &&
+ taa_mitigation != TAA_MITIGATION_TSX_DISABLED) ||
+ mmio_mitigation != MMIO_MITIGATION_OFF ||
+ rfds_mitigation != RFDS_MITIGATION_OFF);
+}
+
static void __init mds_select_mitigation(void)
{
- if (!boot_cpu_has_bug(X86_BUG_MDS) || cpu_mitigations_off()) {
+ if (!boot_cpu_has_bug(X86_BUG_MDS) || cpu_mitigations_off())
mds_mitigation = MDS_MITIGATION_OFF;
- return;
- }

if (mds_mitigation == MDS_MITIGATION_AUTO)
mds_mitigation = MDS_MITIGATION_FULL;
@@ -290,9 +325,29 @@ static void __init mds_select_mitigation(void)
if (mds_mitigation == MDS_MITIGATION_FULL) {
if (!boot_cpu_has(X86_FEATURE_MD_CLEAR))
mds_mitigation = MDS_MITIGATION_VMWERV;
+ }
+}

- setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
+static void __init mds_update_mitigation(void)
+{
+ if (!boot_cpu_has_bug(X86_BUG_MDS) || cpu_mitigations_off())
+ return;
+
+ /* If TAA, MMIO, or RFDS are being mitigated, MDS gets mitigated too. */
+ if (verw_mitigation_enabled()) {
+ if (boot_cpu_has(X86_FEATURE_MD_CLEAR))
+ mds_mitigation = MDS_MITIGATION_FULL;
+ else
+ mds_mitigation = MDS_MITIGATION_VMWERV;
+ }
+
+ pr_info("MDS: %s\n", mds_strings[mds_mitigation]);
+}

+static void __init mds_apply_mitigation(void)
+{
+ if (mds_mitigation == MDS_MITIGATION_FULL) {
+ setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
if (!boot_cpu_has(X86_BUG_MSBDS_ONLY) &&
(mds_nosmt || cpu_mitigations_auto_nosmt()))
cpu_smt_disable(false);
@@ -595,7 +650,6 @@ static void __init md_clear_update_mitigation(void)

static void __init md_clear_select_mitigation(void)
{
- mds_select_mitigation();
taa_select_mitigation();
mmio_select_mitigation();
rfds_select_mitigation();
--
2.34.1